The proposed research is concerned with the intracellular expression and significance of several important antigens that have previously been studied mostly in terms of the cell surface. The major emphasis will be on the H-2, Thy-1, and especially the Tl(thymus-leukemia) antigens of the mouse. The last can be normal thymic differentiation antigens in some strains, and tumor-specific antigens in others. Their appearance (and also that of Thy-1) can be induced in vitro when stem cells from marrow or spleen are caused to differentiate into thymocytes, and their apparent disappearance can be brought about by specific antibody in the absence of lytic complement (antigenic modulation). Our previous work suggests that TL antigens are present in internal cellular organelles, especially mitochondria. We want to document this point further by immunoelectron microscopy of isolated mitochondria and by "cytotoxic" tests on isolated mitochondria (for which we propose to develop the required methodology). The intracellular presence of TL, and the other antigens, will be studied in normal cells, tumor cells, inducible stem cells (prothymocytes), and cells specifically modulated both in vitro and in vivo. If the intracellular location of some of these antigens can be established, this could stimulate investigations of their intracellular function and on mechanisms by which stimuli at the cell surface trigger the differentiation and adaptation of cells.